I would live to build over our double car garage. Then I could get everything in one room. I we ever had enough money to build our own house my quilting room would be 35' x 40' with a 10' x 10' section for the laundry room.

I, too, would go for the bigger room. You will be surprised how fast it will fill up on you. If you have an upstairs, let that be for company. It will be your house so set it up for you & DH. You will also want to consider how long you will be able to handle the stairs. It gets harder every year for me to do the stairs so I have moved a lot of my quilting to the dinning room. I do keep most of my supplies upstairs but only tackle the stairs when I need to switch fabrics. I keep enough downstairs to do at least 4 projects before a upstairs run. Besides, DH would rather I be closer to him. If your downstairs room dose not have a door to hide everything, that would be a good reason to make a quilted curtain to close it off.

My sewing area is in our master bedroom - but keep in mind that my master is something like 24x18. It's a HUGE room and comfortably houses a king bed, a dresser, two bookcases, my desk, a Horn of America Quilter's Dream cabinet that I can open and leave completely open without it getting in the way, a small table for my second machine, a cutting table that's set up at all times, an ironing board also set up all the time, and two end tables by the bed. It may sound crowded but it really isn't, and when I do close down my cabinet and cutting table, it is huge.

That said, I hate it because I am upstairs by myself all the time. If I want to talk to my husband, I have to go downstairs, and I feel somewhat isolated. My 18 yr old son spends a lot of time upstairs in his room when he's home, with the door closed, of course, so that doesn't help. And when he graduates HS in June, gets a job and is going to school and/or leaves home, it will only get worse. I've struggled with what to do about this, because I am lonely up here, and DH will support whatever I decide to do, but the only real place downstairs for me to move into would be the breakfast area. That means we wouldn't have a dining table at all (the formal living/dining is a game room with a huge pool table in the middle of it), and that doesn't bother me overmuch, but my son does sit there to eat when he's home. And the breakfast area isn't all that big ...

If I had a choice of the three situations you offered, I would go with the family room on the main floor. I love my alone time, but in this room, I get a little too much of that. This last weekend, I saw my husband for meals, when I would go downstairs for a glass of water, and about 2 hrs to watch a little tv with him before bed. It was sort of lonely, honestly.

I would prefer mine on the main level of the house to be near the family and not so isolated (as others have said). I have a 20x20 space with plenty of room for all the quilting stuff plus an old sectional from the 1950's parked right in front of the big TV. The kids (teens) will often come watch TV with me while I'm sewing. The TV does need to be pretty loud to hear it but they will suffer for me. Mine is in a detached former maid's quarters and I wish it were part of the house. At least it does have a full bathroom so no running back and forth.

This is more than you asked but you are concerned about people seeing it. The easiest way to make it attractive and keep it clean is to not have too much stuff. I have seen some sewing/craft rooms that look like a bad closet and it's a whole room. Mine has looked like that in the past but I made the choice to have a nice attractive room and keep it clean and it's such a joy now. I still need to work on mine more and get rid of more stuff and it will be even better. I usually work on one project at a time. I have to force myself because it's not in my nature. I also force myself to clean up after each quilt is finished. Again, it's not in my nature to keep it clean - it's something I decided to make myself do and it's totally worth the effort. There are very few times I would be embarrassed because it is a mess.

If you know you are going to be a life-long quilter, then you're going to need a set-up that can grow with you because expand is the name of the quilting game.
First thought...have a basement or 3 car garage for the day that you'll decide you need a long arm. Be sure it's a pleasant place to be.
Now... you need a good cutting surface with room to spread, a sewing surface deep enough to push a quilt around on, a free wall for a design board (that's imperative!) and another wall for stash storage (so something like tall bookcases with doors to close and hide the fabric.)
You can actually use a door and two filing cabinets for cutting and ironing, as well as filing patterns, etc. The main thing is you need SPACE to walk, move easily between your cutting table, sewing machine and ironing board. If you settle for a closet area it will end up becoming very frustrating.
And you won't like being upstairs and away(loft.) I had my own sewing room upstairs away from the family and I literally didn't sew for two years...just didn't like being so isolated.
So, while you're lucky enough to be able to "shop" this for yourself why not sketch out some sewing areas on graph paper and get an idea of just how much space you need? I am in a 12x12 BR and I have a very efficient space but I would BLOW out the wall to the next bedroom and spill over if my husband would let me. Good luck and keep us posted.

I have a lovely quilt room BUT if I were to move, I'd make sure I had these things:

1. Lots of windows for natural light (I currently have no windows. I spend probably 16-20hrs a week in that room because I have a longarm quilting business. I have my every day sewing spewed all over the kitchen - hubby loves that, not! lol). I don't care what anyone says those SAD lights & those light bulbs that duplicate natural light, don't cut the mustard!
2. The more space, the better! (mine is currently about 14' x 16' and altho it IS a lovely space, I never seem to have enough room *giggles*).
3. More built in storage! I have two fantasticly designed closets and some nice cupboard space; BUT you guessed it, I want more!
4. More outlets!
5. Tile floor. (I have carpet and it's a pain in the arse to lug the vacuum up & down the stairs... Oh and the stairs hardly ever get vacuumed lol Guess I could get a 2nd vacuum cleaner.)

With all that said the priority is WINDOWS!!! My space really would be perfect if only I had windows.

I have a room in my basement. (I know, no natural lights! ) I have a fantastic light system and lots of space and a great set up. There is a chair lift that goes down to the space. My basement is basically split into two parts - 1/2 for me, 1.2 for a computer play room and storage. My 1/2 is split into 2 parts - 2/3 sewing, 1/3 pottery. The sewing is 22 x 13, all drywall and painted a lovely green with floors that look like hardwood. Plenty of room for old and new machines, lots of storage thanks to some built ins, a couple great cutting/ironing tables and best of all a bathroom! My husband spends most of his time on that computer, so I might as well be where he is. His room is about the same size - expect he shares it with my granddaughter's toys. I love that I can shut the door and leave everything out and walk away and not have to throw everything into boxes and hurry to pick up the way I did before I had my own place to sew.

I have 2 small bedrooms dedicated to sewing. I have what I need, not near what I want. It is cozy and serves my purpose in the quilting and sewing world.
In my dream world I have a 4x the space. Room to walk around a wide cutting table (Mine now is 48 x 60, but attached to the wall at 48" end. I wanted 5'x5' when we redid it last weekend, but I gave in on that foot in order to stand in front of the ironing board!) Room to have my extra large ironing board away from the wall and plenty of room to walk around it. Room for storing bolts of batting properly and easily managed. An area set up for embroidery, one for serging, one for piecing, one for quilting. A large design wall. An area to display what I have finished. An area for designing and displaying books. An area to put all my fabric and notion cabinets together. Separate area for cabinets of yarn and other crafty things. Proper lighting and windows throughout. Enough outlets without having to use extension cord. Wall space for a tv that can be seen from all sewing chair areas. Place for a microwave, coffee pot, a red retro popcorn machine, and a mini fridge. A bathroom close by. Room for my doggies to lay around and visit often.
I would do this on the lower level. I would think that the weight of it all probably isn't best suited for a loft or above the furnace or a spare bedroom, as I also want a frame someday. I wouldn't want any overnight guests to hear the humming of the machines, as there is midnight sewing.
My sewing is out of eyesight for anyone coming over. I prefer that. I don't feel the need to put anything away when I am in the middle of something and I would certainly forget what I was doing if I had to do that. If you had a large empty basement, think of the possibilities!